<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
      /*div {
        width: 100px;
        height: 100px;
        border: 1px solid red;
      }*/
      #d1 {
        position: absolute; /*绝对定位*/
        /*通过left/right/top/bottom控制元素的位置,参照物为窗口*/
        left: 0;
        bottom: 0;
      }
      #big {
        width: 200px;
        height: 200px;
        background-color: red;
        margin: 100px 0 0 100px;
      }
      #big>div {
        width: 100px;
        height: 100px;
        background-color: green;
        margin: 50px 0 0 50px;
        position: relative;/*设置相对定位,做参照物*/
      }
      #big>div>div {
        width: 50px;
        height: 50px;
        background-color: blue;
        position: absolute;/*设置绝对定位*/
        left: 0;
        top: 0;
      }
    </style>
</head>
<body>

    <div id="big">
      <div>
        <div></div>
      </div>
    </div>

    <!--<div id="d1">div1</div>
    <div>div2</div>
    <div>div3</div>-->
</body>
</html>